1. Field of the Invention
The present invention relates to an apparatus and method for creating high temperature plasmas for enhanced chemical processing of gaseous fluids at a wide range of pressures.
2. Description of the Related Art
There is an urgent need for a practical means for chemical processing of gaseous fluids, such as, pollutants from fossil fuels (NO, SO.sub.2, NO.sub.2, . . . ) which emanate from industrial smokestacks and motor vehicles. Also, there has been no practical means to render harmless toxic chemicals, such as nerve gas, PCB's and dioxins. Additionally, there is a need for a practical means of performing industrial chemical processing at high rates which is impracticable using the present industrial chemical technology due to low reaction rates.
One attempted solution to this problem is to enhance the chemical breakdown of toxic chemicals by irradiating the chemicals, or mixture of the toxic chemicals and a catalyst with microwave energy. The reaction is usually carried out in a closed container at low near vacuum pressures. See, e.g., U.S. Pat. Nos. 3,616,461; 3,619,403; 4,076,606; 4,076,607; and 4,345,983. Specifically, U.S. Pat. No. 4,076,606 discloses a method for decomposing NO.sub.x, or nitrogen oxides, NO.sub.x and oxygen and/or carbon dioxide with microwave irradiation. The technique is relatively efficient at very low, near vacuum pressures. However, the technique does not work well at pressures even as low as 24.1 mm. Hg.; see, Table 3 at col. 10, lines 15-25.
All of the methods and apparatus cited above are impracticable for large scale processing of chemical wastes, or in industrial smoke stacks and automobile exhausts. In order to be practicable, such reactions must be done in an open, flow through, system at atmospheric or higher pressures.
Another approach which claims to provide a practical solution to this problem may be found in U.S. Pat. No. 3,862,043 entitled Pollution Control. U.S. Pat. No. 3,862,043 generally discloses that by interfering with polluting gases by a variety of undisclosed specific frequencies, these polluting gases may be broken down to their constituent parts. The apparatus uses a PYREX.RTM. glass or quartz tube connected to an industrial smoke stack or an automobile exhaust. PYREX.RTM. is a registered trademark of Corning Glassworks, Corning, N.Y. 14830. Around the tube a plurality of sets of windings are each connected to separate generating units which produce various electrical frequencies, each being specifically tuned to breakdown a particular pollutant. However, U.S. Pat. No. 3,862,043 does not disclose how these generating units are individually tuned, nor does U.S. Pat. No. 3,862,043 provide any examples, specific operating parameters or any data to support its claims. Applicant is not aware of any functional apparatus constructed according to the teaching of U.S. Pat. No. 3,862,043.
Another attempted solution to the aforementioned problem is to break down and chemically process these exhaust fumes, toxic pollutants and industrial chemicals in an extremely hot plasma. The production of large volume, high temperature plasmas at low or near vacuum pressures has already been achieved, see, e.g., U.S. Pat. Nos. 3,814,983 and 4,507,588. However, when the pressure is raised to near atmospheric or higher pressures the diffuse, large volume plasmas, produced in accordance with the apparatus and methods cited above, collapse to thin filamentary plasmas which quickly drift toward the microwave source. These filamentary plasmas have been used at atmospheric pressures, e.g., to polymerize monomers on the surface of substrates where the polymer is formed by the plasma heating the monomer, see U.S. Pat. No. 4,521,717; or to fuse silica on the inside of a very thin glass tube used for manufacturing optical fibers, see U.S. Pat. No. 4,125,389.
There have been attempts to isolate the filamentary plasma in a small diameter 5 to 40 mm quartz or PYREX.RTM. glass tube. See, G. Moreau, et al., Microwave Cavity for Atmospheric Pressure Plasmas, J. Phys. E: Sci. Instrum., Vol. 16, Printed in Great Britain (1983) (the "Moreau Article"); P. Taras, et al., Nitric Oxide Plasma Chemical Synthesis in Argon Stabilized Microwave Discharge at Atmospheric Pressure, Acta Phys. Slov. 33, No. 3 (1983) (the "Taras Article"); Y. Arata, et al., Research of a Stationary High Power Microwave Plasma at Atmospheric Pressure, Journal of the Physical Society of Japan, Vol. 40, No. 5 (May 1986) (the "Arata Article"); and U.S. Pat. No. 3,577,207 issued to V. P. Kirjushin, et al., entitled Microwave Plasmatron. However, as in all of the plasma generating apparatus which function at atmospheric or higher pressures, the plasmas produced by these methods drift toward the microwave source. Without any modifications, the plasma settles at one spot inside the glass or quartz tube and eventually cracks or melts the tube. Also, since the plasma is small it allows the flow of gaseous wastes around the plasma and not break down all of the chemical gaseous wastes in the tube. The configurations which utilize a quartz tube cannot be scaled up to utilize high power, high pressures and high flow rates because quartz cannot withstand high pressures, and because of the cracking and melting problems of the quartz tube at high power levels.
The Taras Article discusses attempting to stabilize the plasma inside a glass tube by the introduction of an inert argon gas into the plasma; whereas, the Arata Article attempts to maintain the plasma in the center of the PYREX.RTM. tube by introducing a helical flow of nitrogen gas to form a gas wall between the plasma and the PYREX.RTM. glass tube. Although these plasmas seem to be stable in the very low power small systems discussed in the Taras and Arata Articles, the small microwave plasmas stabilized by inert gases would be impractical for large industrial plants or for smaller plasma producing apparatus which would be installed in automobile exhausts. First, such a system requires a large source of inert gas, such as nitrogen. Also, the use of the inert gas in a glass tube would allow some chemical wastes to flow around the plasma, especially when these wastes are pumped through the apparatus at relatively high pressures present in some industrial processing plants, smoke stacks or in motor vehicle exhausts. In addition, at high power levels the quartz or PYREX.RTM. glass tube would absorb microwave power, heat up and eventually would crack or melt.
The device disclosed in U.S. Pat. No. 3,557,207 produces a low temperature plasma in a spherical or cylindrical cavity. The plasma is produced at atmospheric pressure inside a PYREX.RTM. glass or quartz tube. To maintain the plasma at the center of the tube, the '207 Patent places the wave guide symmetrically around the cavity allowing the electro-magnetic wave to be discharged into the cavity through symmetrical slots surrounding the glass or pyrex tube. The gas is fed into the tube as a turbulent jet which produces a low-pressure region along the axis of the tube to prevent the hot plasma from making contact with the walls of the tube. U.S. Pat. No. 3,557,207 uses a tube having a diameter of about 500 mm to form a plasma column about 300 mm long and 40 mm in diameter. The reported plasma temperatures of 3,000-5,000 degrees are suitable for the limited purposes of U.S. Pat. No. 3,557,207 which are: "conducting chemical reactions of extreme purity, depositing thin films, growing crystals producing powders and other technological purposes." See, Abstract at lines 3-5. However, a 40 mm diameter plasma in a 500 mm diameter tube is not sufficient to fully treat toxic chemicals and pollutants which flow through the tube, past the plasma, at atmospheric or much higher pressures. Also, when highly toxic gases are being purified, such as PCB's or nerve gas, a passage of only a very small amount of the toxic gas past the plasma might be catastrophic.
In addition, with very high power plasmas which would be necessary for treatment of industrial wastes and automobile exhaust, a quartz or PYREX.RTM. glass tube would eventually be coated with deposits from the industrial wastes and automobile exhaust. The deposits would be fused to the surface of the tube by the plasma and block the microwave radiation from entering the tube. Eventually enough microwave radiation would be blocked to prevent the formation of the plasma. Alternatively, the hot plasma and the deposits fused on the tube would eventually eat away the surface of the tube and allow the plasma to drift toward the microwave source, arcing through the waveguide and destroying the microwave source. The major problem with a quartz or PYREX.RTM. glass tube is that at high power levels it absorbs some microwave power. The tube heats up substantially from the absorbed microwave power and eventually cracks or melts.
What is needed is a plasma forming apparatus which allows the pollutants and other toxic chemicals to be fed into the plasma at high rates and at high pressures ranging from about 1 atmosphere (760 Torr) up to a practical limit of about 10 atmospheres (7600 Torr). In order to fully break down pollutants and toxic wastes the plasma must have a cross-sectional area ranging from a little below the diameter of an automobile exhaust pipe up to several feet in diameter when used for industrial treatment of chemical wastes. In addition, such atmospheric pressure and high pressure plasmas require an extremely high electric field, greater than 30 KV/cm at 1 atmosphere which traditionally has required electro-magnetic power density levels in the range of 10-20 watts/cm.sup.3 for a 10,000 degree centigrade plasma. The production of microwave or other electro-magnetic radiation at these power levels is impractical for many applications, for instance, motor vehicles and industrial chemical plants which need power sources that are relatively small and efficient.
High energy plasmas are extremely difficult to contain and stabilize within a resonant cavity. A plasma which drifts in the cavity toward the wave guide and arcs downstream toward the microwave source would immediately destroy the microwave source. Also, as discussed above, the use of a quartz or glass tube to contain the plasma is impractical since the tube would quickly be cracked or melted by the plasma and coated by the hot chemical wastes and exhaust. In addition, use of a turbulent gaseous wall to contain the plasma in the middle of the tube creates the danger of the likelihood of allowing the toxic fluids to pass around the plasma.